An experimental drug for treating severe asthma from GlaxoSmithKline nearly halved the number of attacks suffered by patients with a hard-to-treat form of the disease in a clinical study, boosting hopes for its commercial success. Britain's biggest drugmaker - already a world leader in respiratory medicine - said as a result it planned to move the new drug into final Phase III development before the end of 2012. The injectable antibody treatment mepolizumab is designed to help a small group of people with asthma in which white blood cells called eosinophils cause inflammation of lung airways. It reduces the number of eosinophils by inhibiting an immune system signaling chemical called interleukin-5.